The present invention relates to a method for treating inorganic powder with silylating agents in a batch or continuous manner, whereby the resulting inorganic powder is made hydrophobic.
Inorganic powders, such as fumed silica, are used as thickeners, thixotropics, and reinforcing agents in materials such as inks, resins, rubber, paints and cosmetics. More particularly, they are used to modify the properties, such as the physical, mechanical and thermal properties, of a variety of plastic and elastomeric products such as silicone rubbers.
Inorganic powders are also used as sorbents as disclosed in U.S. Pat. No. 5,037,557. For example, inorganic powders can be used to absorb hydrocarbons, such as oil, from water.
Untreated inorganic powder can have hydroxy groups on its surface that make it hydrophilic. In the case of silica, for example, said hydroxy groups are present as silanol groups. To be effective as a filler in organic systems or a sorbent for organic compounds, the inorganic powder must often be rendered hydrophobic. A method typical of the prior art is disclosed in U.S. Pat. No. 4,554,147. This procedure involves a pneumatically fluidized bed process that requires the use of a quantity in excess of stoichiometric amounts of a treating agent such as a cyclopolysiloxane, long residence times and high operating temperatures. The operating conditions of the prior art processes make treated hydrophobic inorganic powder very expensive. For example, treated fumed silica is one of the most expensive ingredients incorporated in silicone rubber products.
An economically desirable process for rendering inorganic powder hydrophobic would be one that operates at moderate temperatures with short residence times while using modest amounts relatively inexpensive treating agents. The present invention provides such a process.
The treated inorganic powder that results from the method of the instant invention can be substituted in applications that incorporate hydrophobic inorganic powders prepared by methods of the prior art. When the treated inorganic powder resulting from the method of the instant invention is incorporated into a final product, such as silicone rubber, properties of the final product, including physical, thermal and mechanical properties, are substantially unchanged from product that incorporates inorganic powder treated with methods of the prior art.